(ATTN: RECASTS lead; ADDS speculations over his cause of death in last 6 paras) SEOUL, Dec. 19 (Yonhap) -- North Korean leader Kim Jong-il is believed to have died of a massive heart attack as Pyongyang announced, experts in South Korea said Monday, though some raised suspicions of a power struggle and other causes.

North Korea's state media announced that the 69-year-old leader died of a heart attack during a train trip on Saturday, the same illness that killed his father and national founder, Kim Il-sung, in 1994.

The country's Korean Central News Agency (KCNA) said an autopsy confirmed "an advanced acute myocardial infarction complicated with a serious heart shock" as the major cause of death.

North Korean leader Kim Jong-il (L), who died at the age of 69 on Saturday, speaks to his father and national founder Kim Il-sung on a Pyongyang street in 1981. (Yonhap file photo)

Kim's health is believed to have worsened after he suffered an apparent stroke in 2008, although he appeared relatively vigorous in photos and video clips from recent trips to China and Russia and in numerous field trips across the country.

Medical experts in South Korea said what likely claimed his life was not the aftereffects of the stroke but a massive heart attack accompanied by arrhythmia.

"Acute myocardial infarction along with arrhythmia tends to be terminal," said Kang Deok-hyun, a heart specialist at Seoul's Asan Medical Center.

Acute myocardial infarction, commonly known as a heart attack, results from the interruption of blood supply to a part of the heart due to the blockage of a coronary artery, causing heart cells to die. Major risk factors include previous cardiovascular or kidney disease, old age, obesity, excessive alcohol consumption, tobacco smoking and high levels of stress, according to experts.

The late leader was known for his "bad habits" of excessive drinking, smoking and eating since his cosseted childhood, which led to a history of serious ailments including diabetes, stroke and pancreatic cancer. Some speculated that he may have been on kidney dialysis at the time of his death.

His father Kim Il-sung also died of sudden heart failure in 1994 at the age of 82 while appearing hale and healthy.

"Both Kims, who reportedly enjoyed meat diets and were reportedly obese, suffered from cardiovascular disorders. The junior Kim's condition seemed to deteriorate due to fatigue and cold weather," said Park Si-hoon, a heart specialist at Ewha Womans University's Mokdong Hospital in Seoul. "First-aid measures should be followed by immediate treatment either with equipment or medicine within an hour of the symptoms to ensure steady blood flow, which would have not been possible while on a train," said Park Deok-woo of Asan Medical Center.

Kim Jong-un, the late Kim's youngest son and heir apparent, is also suspected of having health problems similar to his predecessors despite only being in his 20s.

Cardiac disease is the third major cause of death among South Koreans, following cancer and cerebrovascular disease, according to government data.

A total of 23,407 people, or 9.2 percent of all fatalities, died of cardiac disorder in 2010.

Despite the North's official announcement, some cast doubt over the cause of the leader's death.

"The fact the communist country made public the news two days after his death could imply that a possible power struggle within the hermit kingdom might have been related to the leader's death," a South Korean expert on North Korea said, requesting anonymity.

Rep. Chun Yu-ok of the ruling Grand National Party also said in a message posted on her Web site, "North Korea bothered to report the autopsy results, which would indicate that suspicions rise within the communist country that Kim's sudden death has some doubtful aspects."

Others pointed out Pyongyang tried to deify its leader by stressing that Kim died from fatigue caused by "his uninterrupted field guidance tour for the building of a thriving nation."

The consensus among government officials and experts here, however, is that there would be no other choice but to believe what Pyongyang said about the death.

"We don't have any clues to say anything different from what North Korea say," a government official said on the condition of anonymity. "There would also be no ways for us as of now to confirm speculations over his death."